This invention generally pertains to mutual coupling cancellation of cylindrical antenna arrays and more particularly to minimizing or cancelling mutual coupling between closely spaced, continuous-slot waveguides without the use of RF absorber material.
Generally, missiles employ microwave antenna arrays for guidance and detonation purposes. These antennas are generally placed at regularly spaced intervals about the circumference of the shroud of a missile. The antennas and shroud of the missile are then covered by a radome. This array of antennas projects a conical beam about the missile. This conical antenna beam detects the target regardless of the angle of approach of the target with respect to the missile.
In present day missiles, multiple antenna systems are employed. These multiple antenna systems project beams in different directions. For example, these directions may include the fore and aft directions. Typical long, continuous-slot waveguide antennas are depicted in U.S. Pat. No. 4,328,502, issued on May 4, 1982, to G. Scharp. These antennas are rectangular waveguides with semi-circular slot antennas cut through one surface of the waveguide.
As previous mentioned, these waveguide antennas are mounted about the periphery of the shroud of a missile. Each beam, fore or aft, is made up of a number of these waveguide antennas to provide total coverage around the missile for signal reception . These antennas are oriented so that the length of the slot of the antenna is along the length axis of the missile.
To achieve multiple beam of coverage with respect to the missile, the waveguide antennas are staggered about the periphery of the missile. That is, the placement of the antennas is about the periphery of the missile. These antennas are alternating aft and fore beam antennas. A common placement of antennas is approximately 60 degrees between antennas included in each one of the beams. Therefore, there are typically six antennas for each beam placed about the periphery of the missile for each antenna beam (fore or aft). Therefore, in a typical fore/aft antenna configuration, there would be twelve antennas regularly spaced about the periphery of the missile.
Mutual coupling between the transmit and receive antennas is a result of surface wave energy from the transmit antenna. The mutual coupling inhibits target detection by the missile.
One solution to this problem is the use of RF absorbing ablating apparatus placed within the radome of the missile and between each of the waveguide antennas. This RF absorbing material would eliminate a portion of the coupling between adjacent antennas. However, with the use of RF absorbing material sufficient coupling is obtained to prevent efficient signal detection by the missile. In addition, the RF absorber weighs approximately two times as much as non-absorber radome materials. As with any flying device, weight is a significant factor in the device's design.
One such RF absorbing ablating arrangement is shown in U.S. Pat. No. 4,748,449, issued on May 31, 1988, to J. Landers, Jr. et al. and assigned to the same assignee as that of the present invention. In addition, RF absorber material significantly reduces the azimuth beam width for continuous-slot antennas.
Further, the RF absorbing apparatus tends to distort the antenna pattern shapes due to the tolerances in the geometrical interfaces between the RF window and the RF absorber material. Further, the portion of the radome containing the RF absorber will ablate much differently than the portion of an unloaded (no absorber) radome. The RF absorber filled radome will tend to flow off of the missile. The unloaded radome material will actually ablate. Therefore, the radome surface becomes uneven which leads to reduced pattern stability.
Lastly, the use of an RF absorber material in a radome greatly increases the difficulty and cost of fabrication of the radome. The RF absorber material must be mixed or interfaced with the RF window material. This adds additional labor and cost.
Accordingly, it is an object of the present invention to provide for cancelling the mutual coupling between antennas of an antenna array without the use of RF absorbing apparatus.
It is a further object of the present invention to provide an environment which insulates a dielectric material from aerothermal environment.